United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/6QQ/SR-97/081 October 1997
Life
Geoffrey M. Lewis and Gregory A. Keoleian
The life cycle design framework was
applied to photovoltaic (PV) module de-
sign. The primary objective of this
project was to develop and evaluate
design metrics for assessing and guid-
ing the improvement of PV product sys-
tems. Two metrics were used to assess
life cycle energy performance of a PV
module: energy payback time and elec-
tricity production efficiency. These
metrics are based on material produc-
tion, manufacturing, and transportation
energies, and were evaluated for sev-
eral geographic locations. An alumi-
num frame is responsible for a
significant fraction of the total energy
invested in the module studied. Design
options to reduce the energy impact of
this and other components are dis-
cussed.
This Project Summary was developed
by EPA's National Risk Management
Research Laboratory, Cincinnati, OH,
to announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Interest in sustainable energy technolo-
gies that are both practical and affordable
has increased with growing awareness of
the environmental and political conse-
quences of fossil fuel and nuclear electric-
ity generation. PV modules, one variety of
which is the subject of this report, offer a
promising alternative to our current de-
pendence on nonrenewable energy tech-
nologies. Photovoltaic modules convert
some of the energy contained in sunlight
directly into electricity without producing
waste or emissions.
This life cycle design project was a col-
laborative effort between the University of
Michigan and United Solar Systems Cor-
poration (United Solar). United Solar is a
joint venture between Energy Conversion
Devices (ECD) of Troy, Michigan, and
Canon, Inc. of Japan. ECD is a leader in
the research and development of thin film
amorphous silicon photovoltaic modules.
Canon is known worldwide as a manufac-
turer of electronic, office, and photographic
equipment.
The United Solar UPM-880 tandem junc-
tion commercial power generation module
was the product chosen for this demon-
stration project. The UPM-880 is currently
United Solar's standard power generation
module and is the most directly compa-
rable with other manufacturers' products.
It employs thin film amorphous silicon as
the photovoltaic material and contains two
identical semiconductor junctions (hence,
tandem). This module has a rated output
power of 22 watts, is 119.4 x 34.3 x 3.8
centimeters in size, weighs 3.6 kilograms,
and has a stabilized conversion efficiency
of 5%.
The UPM-880 represents a point in the
development of thin film PV technology
which has since been surpassed. The op-
portunity to influence this technology im-
provement made the UPM-880 product
system a good candidate for study. United
Solar is exploring innovative applications
of thin film PV technology including incor-
poration of PV into building materials such
as standing seam metal roofing systems
and roofing shingles that have the ap-
-------
pearance of common asphalt shingles.
Roofs, glazings, and facades all become
producers of electricity in addition to per-
forming their traditional structural or archi-
tectural functions when thin film PV
materials are used to coat building sur-
faces. These building-integrated PV appli-
cations are made possible in part by thin
film characteristics such as ruggedness,
flexibility, and low cost.
Life cycle design was developed to more
effectively integrate environmental consid-
erations into product system design. The
product system encompasses material pro-
duction, parts fabrication and assembly,
use, and retirement. Systems analysis
based on the product life cycle offers a
comprehensive approach for guiding im-
provement of photovoltaics and other prod-
ucts.
Objectives
The primary objective of this demon-
stration project was to develop and apply
design metrics for assessing the energy
performance of photovoltaic technologies.
This study was a partial application of the
life cycle design methodology which also
includes the assessment of waste and
emissions throughout the product life cycle.
The scope of this study was limited by the
availability of life cycle inventory data. The
two metrics discussed here are energy
payback time and electricity production
efficiency.
The length of time required for a mod-
ule to generate energy equal to the amount
required to produce it from raw materials
is called the energy payback time. Energy
payback time is frequently used as a per-
formance benchmark for renewable en-
ergy technologies, particularly PV. Fossil
fuel and nuclear electricity generating
plants are not evaluated by energy pay-
back time because they effectively never
pay back. Generating losses and the on-
going need for input energy (as fuel) con-
spire to ensure that fossil fuel plants cannot
generate as much energy as they con-
sume on a primary energy basis.
Electricity production efficiency is de-
fined as the ratio of the total energy pro-
duced by a generating system over its
lifetime to the sum of energy inputs re-
quired for the system's manufacture, op-
eration and maintenance (including fuel),
and end-of-life management. This ratio can
be used to compare all types of renew-
able or fossil fuel based generating tech-
nologies.
Product Description
Over 26 different materials are used in
the production of the UPM-880, 20 of which
are actually incorporated into the finished
product. Module production begins with a
stainless steel substrate which is pro-
cessed in the following steps: washing,
back reflector deposition, amorphous sili-
con alloy deposition, transparent conduc-
tive oxide (TCO) deposition and scribing,
short passivation, grid pattern printing, and
cell cutting. All steps through TCO depo-
sition are continuous processes.
The processed substrate is laminated
inside encapsulation materials which pro-
vide environmental protection while allow-
ing the maximum amount of light
transmission to the active photovoltaic
material. A sandwich of materials is as-
sembled in the following order (from front
to back): Tefzel (a Teflon based polymer),
EVA, the processed substrate, an EVA/
polymer composite layer, and finally a gal-
vanized steel backing plate (Figure 1).
The steel backing plate is laminated to
the material in the rest of the module by
EVA, making its separation during disas-
sembly nearly impossible. The steel back-
ing plate and aluminum frame serve as
structural components only, providing ri-
gidity and mounting points for the module.
Methodology
Scope and Boundaries
Clearly defined boundaries that constrain
data gathering and analysis are critically
important in comparative product system
studies. Results depend directly on bound-
ary definition, which also determines
whether the results may be compared with
those of other studies. This study included
data for raw material extraction and pro-
cessing (for both product and process
materials), transporting processed materi-
als to manufacturing facilities, manufac-
turing, transporting modules to the use
site, and module use. All data were deter-
mined on a per module basis. Not in-
cluded were data on installation or
balance-of-system (BOS) components.
BOS components include mounting and
support structures, tracking hardware (un-
less the array is fixed), wiring and termi-
nals for interconnection of modules in the
array, power inverters to convert the DC
output of the PV module into utility-grade
AC and to interface with the utility electric-
ity grid, energy storage (if the array is not
grid-connected), and labor for installation,
operation and maintenance. Energy used
for the manufacturing facility physical plant
(lighting and space conditioning) and en-
ergy involved in packaging and packaging
materials were also not included.
Data on the end-of-life phase were not
collected since there is no infrastructure
to deal specifically with PV modules. En-
ergy required for or credited from reuse or
recycling options was not considered in
this study, except as discussed in the
Design Implications section below.
Data Collection and Analysis
All energy data were considered on a
per module basis and were converted to
equivalent primary energy (EPE) to ac-
count for losses in conversion and gen-
eration. EPE makes all energy data
functionally equivalent, allowing direct com-
parison. For example, electric energy from
the grid cannot accurately be compared
to natural gas energy without taking into
account production efficiencies for both
electricity generation and natural gas pro-
duction. Ignoring the fuel required to pro-
duce electricity significantly distorts
analysis. To avoid this, the United States
average electricity generating efficiency
ratio of 0.32 was used to convert electric-
ity to EPE for this study.
Materials
Published data for extracting and pro-
cessing raw materials (material produc-
tion energy) were not available for all
materials used to produce the UPM-880.
Estimates were made for some materials
based on discussions with industry
sources. For materials manufactured by a
small number of firms, energy data are
usually considered proprietary. In these
cases, we substituted data for similar ma-
terials or processes.
United Solar provided a bill of materials
for the UPM-880 with all data items on a
per module basis. United Solar provided
data on supplier location and utilization
Processed substrate
EVA / polymer composite
Steel backing plate
Figure 1. Laminated module, edge cross section.
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efficiency for each material as well, allow-
ing a calculation of the actual amount of
material incorporated into a module and
the amount of waste material per module.
The amount of energy expended to
transport materials to United Solar facili-
ties was calculated using distances deter-
mined from the location data, information
on transportation energy requirements in
units of energy per weight-distance (Btu
per ton-mile), and material weight data
from the bill of materials. It was assumed
that a diesel tractor trailer was used for all
overland transportation and an ocean
freighter was used for all overseas trans-
portation.
Manufacturing
There are three components of manu-
facturing energy: processing energy, en-
ergy in process materials, and energy for
transportation to the use site. Manufactur-
ing process energy data were provided,
on a per module basis, by United Solar.
Process materials were handled by the
same method discussed above for prod-
uct materials. In most cases, however,
data on material production energy for
process materials were not readily avail-
able from the literature. The amounts of
process materials used per module were
low relative to most product material in-
puts, so process materials were assumed
to contribute a negligible amount to total
module energy requirements.
Use
Use phase data for the energy payback
time calculation consist only of insolation
at the module location and the module's
conversion efficiency (module size is a
constant, equal to 0.372m2). Insolation
data, as direct normal solar radiation in
watt-hours per square meter per day, were
taken from the National Renewable En-
ergy Laboratory's (NREL) online computer
database and converted to units of kilo-
watt-hours per square meter per year. Data
were taken for three cities of interest: De-
troit, Michigan, near United Solar and the
University of Michigan; Boulder, Colorado,
near NREL; and Phoenix, Arizona, a loca-
tion generally considered to be an excel-
lent site for PV use. These three cities
approximately span the range of insola-
tion available in the continental United
States, from a low of around 1200 kWh/
m2/yr in Detroit to around 2000 kWh/m2/yr
in Boulder to a high of around 2500 kWh/
m2/yr in Phoenix.
Life Cycle Metrics
Material production energy includes en-
ergy for raw material extraction, process-
ing, and transportation. These data were
gathered in megajoules per kilogram. Be-
cause material production energy data vary
over a wide range, low and high values
were used in separate calculations, re-
sulting in two values for each energy met-
ric. The energy used to transport one
module worth of materials to manufactur-
ing facilities was then calculated and added
to the material energy.
Energy data for each module manufac-
turing process step were gathered on a
per module basis. This energy was all in
the form of electricity and was converted
to equivalent primary energy as discussed
above. Transport to the use site was as-
sumed to be by diesel tractor trailer.
Calculating energy generated by a mod-
ule in use requires data for its stabilized
conversion efficiency and area, along with
average insolation where it will be installed.
Once the energy generated by a module
was known, all data necessary to calcu-
late energy payback time and electricity
production efficiency metrics were avail-
able.
Two other metrics, life cycle conversion
efficiency and life cycle cost, are discussed
in the full report from this project.
Energy Payback Time
Payback time in years was calculated
by dividing the total amount of energy
used to manufacture a module from raw
materials, install and operate it over its
lifetime, and deal with end of life disposi-
tion by the amount of energy a module
generates in a year using Equation 1. The
variables in this equation are defined as
follows: Emat = energy to extract, process,
and transport raw materials to the manu-
facturing facility; Efab = energy to fabricate
a module from raw materials and trans-
port it to the use site; Einst = energy re-
quired for module installation (assumed to
be 0); Eelm = energy required for any end-
of-life management activity (assumed to
be 0); Egen/yr = energy generated by a
module in one year; and E0&m/yr = energy
used annually for operation and mainte-
nance (assumed to be 0).
lifetime (Eelm), using Equation 2. Eiom and
Eelm were assumed to be zero for this
analysis; in actuality both are likely to be
small numbers.
Payback time
+ E
eim
-gen'
(1)
Electricity Production Efficiency
Electricity production efficiency is cal-
culated by summing the energy pro-
duced by a generating system over its
lifetime (Egen (lifetime)), and dividing it
by the sum of the energy inputs re-
quired to manufacture (Emat + Efab), in-
stall, operate and maintain (Eiom, which =
Einst + (module lifetime) (E0&m/yrj), and dis-
pose of or reclaim it at the end of its
Electricity production efficiency
Egen (lifetime)
Emat+Efab + Eiom + Eelm
(2)
Electricity production efficiency was cal-
culated for the same geographic locations
as payback time. Two possible module
lifetimes, 10 and 25 years, were chosen
to demonstrate how this variable effects
the metric (the UPM-880 is currently
warranted for 10 years).
Electricity production efficiency is pre-
sented as a ratio. A system that gener-
ates more energy than is required to
produce it would have an electricity pro-
duction efficiency greater than unity and
could be considered to be a sustainable
system.
Results and Discussion
Life Cycle Data
Energy data for production and trans-
port of product materials are shown on a
per module basis in Table 1, sorted from
highest energy at the top to lowest at the
bottom. When more than one material is
required for a function, it is noted as "vari-
ous" in the material column. Notice also
that there are two totals at the bottom of
the table, one for a standard module and
one for a frameless module. This classifi-
cation highlights the impact of the alumi-
num frame on the energy requirements
for the UPM-880.
Energy required for manufacturing, con-
verted to equivalent primary energy (EPE),
is shown in Table 2. These data were
gathered at United Solar by measuring
electrical consumption of the respective
machines for the amount of time neces-
sary to process one module. The bulk of
this energy is invested in processes that
require elevated temperatures for a long
period of time (encapsulation) or at greatly
reduced pressure (all of the deposition
steps).
Life Cycle Metrics
Energy Payback Time
Energy payback time results are pre-
sented in Table 3. Module production en-
ergy summarizes the material, transport,
and manufacturing energy discussed in
Tables 1 and 2 for both the standard and
frameless cases. Table 3 presents energy
payback times in years for various loca-
tions and module conversion efficiencies.
Energy generated per year is calculated
as the product of insolation, conversion
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Table 1. Product Constituent Material Production Energy, in MJ
Function
Frame
Encapsulation
Substrate
Backing plate
Deposition materials
Busbar
Back reflector
Grid
TCO
Material
aluminum
various
stainless steel
steel
various
various
various
various
various
Low Case
196.0
84.0
58.7
9.7
7.7
0.8
0.2
*
*
High Case
566.1
114.8
73.0
65.4
7.7
3.6
0.7
*
*
Transport
7.8
7.7
3.9
6.1
0.1
0.1
*
*
*
% Module
Mass
38.0
25.2
11.4
24.8
*
*
*
*
*
Standard, total material energy
Frameless, total material energy
357.1
161.1
831.4
265.4
25.5
17.7
* Negligible amount, <0.05.
Standard - low energy case uses lowest reported data and assumes 70% primary / 30% secondary frame material;
high uses the highest available data and assumes frame is 100% primary aluminum .
Frameless - low and high cases reflect the range of values reported in the literature.
Source: Appendix B in [7].
Table 2. Manufacturing Equivalent Primary Energy (EPE)
Process Step EPE (MJ)
% of Total
Encapsulation
Amorphous SI alloy deposition
TCO deposition
Back reflector deposition
Substrate wash
TCO etch
Short passivation
Grid pattern screen print
Testing and packaging
Total process energy
56.2
37.9
32.7
30.3
23.1
7.0
7.0
7.0
*
201.2
28.0
18.8
16.3
15.0
11.4
3.5
3.5
3.5
*
100.0
* Negligible amount.
efficiency, and module size. Energy pay-
back time in years is calculated as mod-
ule production energy (in kWh) divided by
energy generated per year. The conver-
sion efficiency of the UPM-880 is cur-
rently around 5%, but energy payback
times were also calculated for a conver-
sion efficiency of 8% to illustrate the effect
of efficiency improvements on payback
time. United Solar has produced a proto-
type module with a 10% conversion effi-
ciency and is currently translating this
technology into production.
Our methodology results in payback
times higher than previously reported.
Srinivas reports payback times for 5% ef-
ficient amorphous silicon modules pro-
duced in batch production facilities outside
North America. His results ranged from
2.18 years for a frameless module to 2.6
years for a module framed with plastic
and glass using an insolation level roughly
equivalent to our Detroit case. Hagedorn
estimates a payback time of 3.5 years for
a 5% efficient module framed with plastic
and glass produced in a proposed facility.
Construction and material factors in both
of these studies seem to indicate modules
with shorter lifetimes than the UPM-880.
Payback times calculated in this study
should be compared with others published
in the literature only if differences in the
assumptions, data, and methodologies are
carefully considered.
Electricity Production Efficiency
Electricity production efficiency results
are presented in Table 4. Module produc-
tion energy is identical to that noted in
Table 3 and the same three locations are
used, although the number under the lo-
cation now represents the amount of elec-
tricity generated per year by a module at
5% conversion efficiency. Energy gener-
ated over a module's lifetime is the prod-
uct of electricity generated per year and
module lifetime. Electricity production effi-
ciencies were calculated with Equation 2.
Note that the high electricity production
efficiency value for each case results from
the low module production energy value,
and vice versa, and also that values less
than unity result from module production
energy being greater than energy gener-
ated. For comparative purposes, the United
States electricity grid has an average elec-
tricity production efficiency of 0.32.
Design Implications
Two components of the UPM-880 pho-
tovoltaic module offer major opportunities
for improved design: the aluminum frame
and EVA encapsulant.
Energy invested in the aluminum frame
consists of material production energy and
energy required to extrude and anodize
the frame parts. Material production en-
ergy can be reduced by using a higher
proportion of secondary (scrap) aluminum
to primary material, or by using a differ-
ent, less energy intensive material. A
higher proportion of secondary material
might cause a decrease in the frame's
surface quality, but as long as its struc-
tural properties and lifetime remain unaf-
fected, cosmetic imperfections should be
tolerable. Use of the module in applica-
tions not requiring a frame obviates the
material selection process for this compo-
nent and also eliminates significant en-
ergy investments.
Reusing the aluminum frame is another
method of reducing energy investment. In
the current design, the frame is easily
separable and can be used on another
module with minimal processing besides
transportation to the production facility. The
impact of reusing the frame on energy
metrics is dramatic, because the frame
represents between 34 and 53 percent of
the total module production energy (be-
tween 55 and 68 percent of total material
production energy). Reusing the frame
once halves its energy contribution and
reusing it twice drops the energy cost to
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Table 3. Energy Payback Time Calculations
Payback Times
Location and Conver.
Insolation Eff., %
Detroit, Ml
1 202 kWh/m2/yr
Boulder, CO
1 974 kWh/m2/yr
Phoenix, AZ
2480 kWh/m2/yr
5
8
5
8
5
8
Energy Gen/
Year (kWh)
22.3
35.7
36.7
58.7
46.1
73.7
Standard
low
7.5
4.7
4.5
2.8
3.5
2.2
Standard
high
13.4
8.4
8.1
5.1
6.4
4.0
Frame-
less
low
5.0
3.1
3.0
1.8
2.3
1.4
Frame-
less
high
6.3
3.9
3.7
2.3
2.9
1.8
Standard: module production energy is: material production + manufacture + transport = 162.2 kWh (583.8 MJ) low
case; = 293.9 kWh (1058.1 MJ) high case.
Frameless: module production calculated as above = 105.6 kWh (380.0 MJ) low case; = 134.5 kWh (484.3 MJ) high
Table 4. Electricity Production Efficiency Calculations*
Electricity Production Efficiencies
Location and
Generation
Detroit, Ml
22.3 kWh/yr
Boulder, CO
36.7 kWh/yr
Phoenix, AZ
46.1 kWh/yr
Module
Life (yr)
10
25
10
25
10
25
Standard
low
0.75
1.87
1.24
3.09
1.56
3.91
Standard
high
1.33
3.33
2.23
5.57
2.83
7.07
Frameless
low
1.60
3.99
2.68
6.69
3.40
8.51
Frameless
high
2.01
5.03
3.39
8.49
4.33
10.83
"Assumes 5% module conversion efficiency, includes module transport energy: Detroit, 19.31 MJ; Boulder, 8.97MJ;
Phoenix, 3.01 MJ.
Standard: module production energy is: material production + manufacture + transport = 162.2 kWh (583.8 MJ) low
case; = 293.9 kWh (1058.1 MJ) high case.
Frameless: module production calculated as above = 105.6 kWh (380.0 MJ) low case; = 134.5 kWh.
Table §. Energy Metrics for Frame Reuse
Location and Metric
Number of Uses
Detroit, Ml
low energy payback time (yr)
high energy payback time (yr)
low electricity production efficiency
high electricity production efficiency
Boulder, CO
low energy payback time (yr)
high energy payback time (yr)
low electricity production efficiency
high electricity production efficiency
Phoenix, AZ
low energy payback time (yr)
high energy payback time (yr)
low electricity production efficiency
high electricity production efficiency
7.5
13.4
0.8
1.3
4.5
8.1
1.2
2.2
3.5
6.4
1.6
2.8
6.8
10.4
1.0
1.5
3.9
6.1
1.7
2.6
3.0
4.7
2.1
3.4
6.8
9.7
1.0
1.5
3.8
5.5
1.8
2.7
2.8
4.2
2.4
3.5
Assumes 5% module conversion efficiency; 10 year lifetime; includes transportation energy.
one third of the single-use value. Table 5
contains energy metrics calculated for vari-
ous levels of frame reuse. These results
assume a module with 5% conversion ef-
ficiency and include energy to transport
the module back to the manufacturing fa-
cility for each frame reuse. Transportation
energy was assumed to be the same for
each use of the frame; distance from the
module disassembly facility to United So-
lar was the same as the distance from the
frame manufacturer to United Solar.
The useful life of a photovoltaic module
is a primary design parameter, as indi-
cated in Table 4. EVA encapsulant fre-
quently determines a module's useful life
as it either degrades in optical quality or
moisture permeability. Formulations of EVA
have evolved to the point where browning
is no longer the concern it once was, but
moisture permeability remains a main de-
terminant of module lifetime. In addition,
the current formulation of EVA requires
relatively high energy for lamination. A
formulation with a quicker cure time and/
or a lower cure temperature would reduce
this process energy requirement.
The one other likely candidate for com-
ponent reuse is the steel backing plate. In
the current design, the backing plate is
bonded to the module in the laminating
press by a layer of EVA. If this layer of
EVA could be deleted from the module, it
would greatly facilitate disassembly and
reuse or recycling of the backing plate
while reducing material energy require-
ments. However, our calculations revealed
that eliminating one layer of EVA and re-
using the steel backing plate had only an
incremental effect on values of the metrics,
especially compared to the effect of reus-
ing the frame. Even so, deleting a layer of
EVA does facilitate manufacturing and end
of life management and can reduce cost.
Conclusions
The application of the life cycle design
framework offered many useful insights
for enhancing the energy performance of
photovoltaic technology. Life cycle energy
analysis highlighted the energy contribu-
tion of individual life cycle stages, process
steps, parts and components, and spe-
cific materials. The project team devel-
oped metrics to guide improvement of
photovoltaic devices and to assess how
sustainably these devices generate elec-
tricity. One of these metrics, electricity pro-
duction efficiency, was discussed.
The metrics presented in Tables 3 and
4 demonstrate the relative significance of
geographic locations with higher insola-
tion and the aluminum frame. The best
results are obtained with frameless appli-
cations in areas of high insolation, but
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either increased insolation or reduced mod-
ule production energy are beneficial indi-
vidually. For example, comparisons of
standard and frameless modules indicate
that the frame approximately doubles en-
ergy payback time and reduces electricity
production efficiency by about half. The
beneficial effect of increased module life-
time is also clearly demonstrated in Table
4.
Electricity production efficiency is a pow-
erful metric for comparing photovoltaic
technology with other systems for gener-
ating electricity because it puts all sys-
tems on an equivalent basis. To meet a
definition of sustainability, an electricity
production efficiency greater than unity is
necessary: this enables the device to pro-
duce sufficient energy over its lifetime to
at least reproduce itself (the current United
States electricity grid efficiency is 0.32).
All but one of the cases presented in
Table 4 show efficiencies greater than
unity; most are substantially higher.
The energy investment in a conven-
tional power plant is generally neglected
in life cycle energy analysis because it is
assumed to be small relative to fuel en-
ergy inputs. This study shows that energy
investment in the "power plant" for photo-
voltaic devices is substantial relative to
their energy generating capacity and can-
not be neglected. A comprehensive and
fair comparison of PV and conventional
generating systems would involve enu-
merating all terms in (2) (including any
storage necessary for the PV system) and
other environmental impacts such as air
emissions and waste for both systems.
A simple but important conclusion from
Table 5 is that increasing the number of
module components that can be reused
and the number of times they are reused
significantly improves energy metrics. Re-
using the aluminum frame will yield by far
the greatest improvement in energy
metrics; reusing other components affects
the metrics only incrementally and may
not be worth additional effort from an en-
ergy standpoint.
Energy payback time should be a criti-
cal factor in deciding whether or where to
deploy photovoltaic modules, although cost
is usually the sole criterion for these deci-
sions. Accurate comparison between our
values of this metric and other studies
requires careful consideration of differ-
ences in methodology or data. This study
is based on actual data from an operating
production facility; many other studies use
theoretical calculations which tend to im-
prove metrics.
Photovoltaic technology development
focuses primarily on increasing conver-
sion efficiency and reducing cost. How-
ever, energy payback time and electricity
production efficiency add valuable perspec-
tives for guiding photovoltaic technology
development. Energy payback time can
be used for strategic planning and deci-
sion making when all assumptions are
considered. Electricity production efficiency
is a more comprehensive metric because
it assesses the performance of a generat-
ing system over its entire lifetime. This
metric should also be used by designers
for product material selection and process
design. In addition, PV manufacturers, util-
ity companies, policymakers, and the pub-
lic should use this metric to make accurate
comparisons between generating technolo-
gies.
The properties of amorphous silicon thin
film technology seems to make it a natu-
ral fit in building-integrated PV applica-
tions such as glazing and sheathing
materials and standing seam metal roof-
ing.
The full report was submitted in partial
fulfillment of Cooperative Agreement num-
ber CR-822998-01-0 by the National Pol-
lution Prevention Center at the University
of Michigan under the sponsorship of the
United States Environmental Protection
Agency.
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Geoffrey M. Lewis and Gregory A. Keoleian are with the University of Michigan,
Ann Arbor, Ml 48109-1115.
Kenneth R. Stone is the EPA Project Officer (see below).
The complete report, entitled "Life Cycle Design of Amorphous Silicon Photovol-
taic Modules," (Order No. PB97-193106; Cost: $21.50, subject to change) will
be available only from
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at
National Risk Management Research Laboratory
U. S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/6QQ/SR-97/081
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